The present invention relates to an objective lens supporting device for an optical pickup for use with an optical disc player such as a compact disc player and a video disc player, and more particularly to an objective lens supporting device for an optical pickup including a lens holder operatively supported by supporting rods formed of elastic material in such a manner as to be driven in a focussing direction and a tracking direction.
FIG. 7 is a perspective view of a conventional objective lens supporting device for the optical pickup which device is similar to that disclosed in Japanese Utility Model Application No. 62-000787, and FIG. 8 is a cross section taken along the line VII--VII in FIG. 7.
Referring to FIG. 7, reference numeral 1 designates an objective lens opposed to an optical disc (not shown). The objective lens 1 is held by a lens holder 2. The lens holder 2 is finely movably supported by four supporting rods 3 to a fixed member 4. Each supporting rod 3 is formed of elastic material such as rubber and elastomer.
A focussing coil 6 is wound around a rectangular outer periphery of the lens holder 2, and tracking coils 7 are fixed to an outer periphery of the focussing coil 6. A pair of magnets 9 retained by yokes 8 are located outside the tracking coils 7 in such a manner as to be opposed to the focussing coil 6 and the tracking coils 7.
In the conventional objective lens supporting device for the optical pickup as mentioned above, the lens holder 2 is driven by the current flowing in the focussing coil 6 and the magnetism of the magnets 9 to thereby deform the supporting rods 3 and effect the focussing correction in a direction (Y-direction) of an optical axis of the objective lens 1. In such a focussing correcting operation, a detecting beam to be irradiated from the objective lens 1 onto an information recorded surface of the optical disc is corrected to be always focussed on the recorded surface. Further, the lens holder 2 is driven by the current flowing in both side portions of the tracking coils 7 and the magnetism of the magnets 9 to thereby deform the supporting rods 3 and effect the tracking correction in a direction (X-direction) along the information recorded surface of the optical disc. In such a tracking correcting operation, a spot of the detecting beam to be irradiated from the objective lens 1 onto the information recorded surface of the optical disc is corrected to precisely scan an information track of the recorded surface.
Generally, in such an objective lens supporting device for the optical pickup as mentioned above wherein the lens holder 2 is driven to elastically deform the supporting rods 3 and effect the correcting operation, the X-direction and Y-direction oscillation characteristics of an oscillating system consisting of the objective lens 1, the lens holder 2 and the supporting rods 3 have a large influence on the tracking correcting operation and the focussing correcting operation. In other words, this type lens supporting device is required to have the elastic characteristics of the supporting rods 3 upon deformation in the X-direction substantially identical with those in the Y-direction. To this end, it is desirable to make the spring constant of the supporting rods upon deformation in the X-direction identical with that in the Y-direction. If the spring constant in the X-direction differs from that in the Y-direction, and the elastic characteristics in the X-direction therefore differ from those in the Y-direction, the lens holder 2 tends to be operated in the direction where the spring constant of the supporting rods 3 is smaller, causing a problem of auxiliary resonance.
In manufacturing the supporting rods 3 to have a uniform sectional area in the longitudinal direction as shown in FIG. 7, there is a possibility of torsion and strain generating upon injection molding, for example, of the supporting rods 3. In the injection molding, there is a possibility that unevenness is formed on the surface of the rods 3 by the depression of a knockout pin when the supporting rods 3 are removed from a metal mold. As a result, even in forming the supporting rods 3 each having a square section, the shape of the section is rendered ununiform due to strain as shown in FIG. 8. Further, the direction of the section in each sectional position along the longitudinal direction is varied by the torsion of the supporting rod. As a result, a geometrical moment of inertia Ix with respect to a reference axis X intersecting a primary axis O differs from a geometrical moment of inertially with respect to a reference axis Y intersecting the primary axis O in each section, thus causing a difference in the spring constant of the supporting rod 3 between in the tracking direction (X-direction) and the focussing direction (Y-direction). As a result, the elastic characteristics, namely, the oscillation characteristics in the X-direction and the Y-direction are not balanced to cause the auxiliary resonance in the correcting operation.